Add rosidl buffer backend metadata to topic endpoint info

[nvcyc]

Description

Add rosidl buffer backend metadata to rmw_topic_endpoint_info_t so graph introspection APIs can carry backend support information for publishers and subscriptions.

Is this user-facing behavior change?

Yes. This enables higher-level ROS graph tools to expose buffer backend support information for topic endpoints.

Did you use Generative AI?

Yes. GPT-5.5 in Cursor was used to help draft changes in this pull request.

Additional Information

This PR only adds the RMW-level data model.
RMW implementations and client libraries need separate changes to populate and expose the metadata.

[Yadunund]

@nvcyc is there a test in ros2cli that we need to update as well?

See test failures across RMW impls: https://ci.ros2.org/job/ci_linux/29339/testReport/junit/ros2topic.ros2topic.test/test_cli/test_cli/

[fujitatomoya]

the struct previously held only const char * owning pointers, but it now holds a rcutils_string_map_t whose impl is a heap pointer with its own fini. any code path that copies an rmw_topic_endpoint_info_t by plain struct assignment or memcpy now aliases that impl between two structs.
the existing const char * members have the same hazard in principle, but the codebase already established a deep-copy discipline for strings.
i'd suggest a rmw_topic_endpoint_info deep-copy helper, or at minimum confirmation that no array/element copy in the current code and companion PRs.